--- title: "Equilibrium potentials and what sets the resting voltage" description: "An ion's equilibrium potential depends on its charge, temperature and concentrations inside and outside the cell. A typical resting neurone lies near the potassium equilibrium potential because its me" canonical: https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1 source: https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Equilibrium potentials and what sets the resting voltage An ion's equilibrium potential depends on its charge, temperature and concentrations inside and outside the cell. A typical resting neurone lies near the potassium equilibrium potential because its membrane is highly permeable to potassium. Other ion currents and the sodium-potassium pump also contribute. Subject: Science · Area: The Human Body · Ages 18 to 19 Page: https://lightmysky.com/learn/science/equilibrium-potentials-and-what-sets-the-resting-voltage-mt_mRT_5vDEO1 ## Ready when they can - Calculates an equilibrium potential from inside and outside concentrations. - Predicts how the resting voltage shifts when external potassium is raised. - Explains why the resting membrane sits near the potassium value but not exactly at it. ## Lesson: The voltage each ion wants You have met a neurone's resting voltage. Membrane voltage means inside relative to outside. An ion's equilibrium potential, E_ion, is the voltage where its concentration gradient and electrical force balance. Its net passive flow through open channels is zero, though individual ions can cross both ways. For K+ concentrated inside, diffusion favours outward movement while a negative inside attracts it inward. The sodium-potassium pump uses one ATP to move three Na+ out and two K+ in, against their concentration gradients. It maintains high sodium outside and high potassium inside. Each cycle also exports one net positive charge, tending to make the inside more negative. Pumps maintain gradients over time; channels allow the fast passive ion flows that change voltage. At 37°C, use E_ion = (61.5 mV/z) log10(c_out/c_in). This Nernst equation uses concentrations to approximate ion activities. c_out and c_in are outside and inside concentrations in matching units. z is signed ion charge: +1 for K+ or Na+, -1 for Cl-. Divide outside by inside, take the base-10 logarithm with a calculator, then multiply by 61.5/z. The coefficient changes with temperature. *(drawing: For K+ at 37°C, use 1 mM outside and 100 mM inside. Its charge is +1, so multiply the logarithm by 61.5 mV.)* A typical resting neurone is more permeable to K+ than to Na+, so its voltage, V_m, lies near E_K. Sodium entry usually shifts V_m toward the more positive E_Na. Other ion currents and pump current also contribute, so V_m need not equal E_K. With no applied current, a steady V_m means total membrane current, including pump current, is zero. Individual ion currents can still flow. Keep inside K+, temperature, other ion concentrations and permeabilities fixed. Raising outside K+ increases c_out/c_in, making a negative E_K less negative. A potassium-dominated resting voltage also becomes less negative. At 37°C, doubling outside K+ raises E_K by 61.5 log10(2), about 18.5 mV. V_m need not rise by exactly that amount. Lowering outside K+ shifts E_K in the negative direction. Permeability means how readily an ion crosses the membrane. It affects V_m but is not an input to the Nernst equation. With concentrations and temperature fixed, opening more K+ channels leaves E_K unchanged while shifting V_m toward E_K. Increasing Na+ permeability shifts V_m toward E_Na. These predictions give a direction; they do not calculate an arithmetic average of equilibrium potentials. **Recap.** Calculate an ion's equilibrium potential from charge, temperature and its concentration ratio. Resting voltage also depends on permeability, other ion currents and pump current. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [The Resting Potential and the Action Potential](https://lightmysky.com/learn/science/the-resting-potential-and-the-action-potential-mt_0sh4w30ocz) ## Optional connections - [Co-transport and Coupled Movement Across Membranes](https://lightmysky.com/learn/science/co-transport-and-coupled-movement-across-membranes-mt_pzQ8n3FWNV) ## Opens up - [Channel Kinetics and the Ionic Basis of the Spike](https://lightmysky.com/learn/science/channel-kinetics-and-the-ionic-basis-of-the-spike-mt_47-LDzgcKp)